1. Field of the Invention
The present invention is concerned with an objective lens for a camera, comprising diaphragm disposed at the front of the lens and an optical assembly disposed behind the diaphragm.
2. Description of the Prior Art
French Pat. No. 2 321 713 filed Aug. 22, 1975 discloses a camera whose essential characteristic is that the iris diaphragm of its objective lens, that is to say the diaphragm of the objective lens which delimits the incident beam and thus conditions the aperture number, is situated in front of the objective lens.
It has become clear that this arrangement, originally devised in order to facilitate and simplify the practical implementation of underwater cameras, can with advantage by applied to cameras for discreet surveillance applications, including still cameras, movie cameras and video cameras, whether the need is for surveillance in an air or water medium, for monitoring the presence of persons therein.
A condition for the success of any surveillance system is that the camera by virtue of which it is exercised be hidden from view of the greatest possible extent.
If, as is usual, the iris diaphragm is disposed inside the objective lens, the entry lens elements thereof are necessarily of relatively large diameter, making it difficult to hide them.
This does not apply if, as described in the aforementioned French patent, the iris diaphragm of the objective lens is situated at the front of the lens.
The entry lens elements of the objective lens may then have a relatively small diameter, and the corresponding camera may thus be easily hidden behind a partition or closet door or any appropriate support, a small hole in the front surface, difficult to see, constituting the iris diaphragm of the objective lens of the camera thus hidden.
In practice, apart from this capacity for being hidden, it is desirable for other constraints to be met to obtain the widest possible application.
First of all, as already mentioned, it is desirable for the objective lens to be equally suitable for use in air or water.
It is also desirable to obtain a relatively large distance between the lens and the corresponding focal image plane to provide for the installation of any necessary accessory device, such as a shutter, reflex mirror or filter, for example.
These requirements are mutually contradictory, and are difficult to reconcile in practice.
In the aforementioned French patent, the objective lens described is in practice suitable only for underwater cameras, and the distance between it and the focal image plane is relatively small.
A general objective of the present invention is to provide an objective lens which provides a particularly satisfactory solution to the problem of meeting the stated requirements.